Inhalers are commonly used for delivery of a wide range of medicaments. The inhaler holds a canister of medicament, the canister being actuated by compression to deliver a dose of medicament through a mouthpiece to a user.
Canisters for holding a suspension or solution of a pharmaceutical substance in a propellant under pressure are well known. One such known canister to be used in an inhalation device comprises a can coupled with a valve. The valve consists of a body comprising a chamber, a valve stem (which extends from a head of the body) and a metering chamber, selectively communicable by the valve stem to the atmosphere via an L-shaped conduit within the valve body. The valve stem is axially displaceable between a first, extended position in which the metering chamber, and hence the canister, is closed to the atmosphere (since the L-shaped conduit is disposed wholly outside the metering chamber), and a second, depressed position, in which the metering chamber is in open communication with the outlet provided by the L-shaped conduit in the valve stem and through which a metered dose of propellant containing pharmaceutical substance is delivered. The valve stem is inserted into a stem body component, which is part of the mouthpiece and housing of the device. The patient applies a compressive force between the base of the canister, held in an inverted position in the housing, and the stem body which forces the valve stem into the canister. When the valve stem has been depressed sufficiently to open the metering channel, the inhaler will discharge a single dose. This is known as the firing point. This technology is well known and described in many parts of the prior art. For example in Respiratory Drug delivery, Ed Peter Byron, CRC press, 1991, and in Lung Biology in Health and Disease, Vol 94, Inhalation Aerosols, A J Hickey, Publisher Marcel Decker, 1996 and in Pharmaceutical Preformulation and Formulation, Ed Mark Gibson, Chapter 10, Inhalation Dosage Forms, IHS Health Group, 2001.
It is important for the patient to be able to ascertain the level of medicament remaining in the canister. This requirement is met by the use of a dose counter accessory either attached to or part of the inhaler device. It is obvious that this counter must be accurate in that over-counting results in medicament being left in the inhaler unnecessarily resulting in more repeat prescribing with their resultant costs and inconvenience to patient. It also means that in an emergency situation the patient may not use the inhaler in the belief that it is empty when it is not. Conversely, undercounting could result in the inhaler being empty of medicament before the patient has a replacement, which could be dangerous for the patient and reduce patient compliance. Regulatory guidelines require the over and undercounting errors to be minimised.
An inhaler traditionally contains from 120 to 200 doses (shots) so any counter must be reliable, accurate and easy to read. There are many examples of dose counters in the prior art, some located near the valve region of the canister and some attached to the base of the canister which due to the fact that the canister is in an inverted position in the inhaler mouthpiece means that the counter is on the top of the assembled inhaler and can be easily viewed.
In U.S. Pat. No. 6,446,627 a dose counter attached to the base of the actuator is described. The counter measures the displacement of the top of the canister relative to the valve stem. The counter is not attached to the canister and therefore a problem is that the user can swap canisters between actuators and that would cause the indicator of the counter to display the incorrect number of doses remaining in the canister. Furthermore a problem with the inhalation device according to U.S. Pat. No. 6,446,627 is that the actuator and counter assembly are difficult to clean and are vulnerable to damage with water and cleaning products.
In U.S. Pat. No. 4,817,822 a counter attached to the base of the canister, i.e. on the top of the inhalation device, is disclosed. One embodiment of this inhalation device measures the displacement of the canister base relative to the actuator body. A problem with the counter disclosed in U.S. Pat. No. 4,817,822 is miscounting since the counter relies on the displacement of the canister and the canister length due to manufacturing variables is a length with a variable tolerance. A large tolerance of the canister length has the potential to cause miscounting.
In U.S. Pat. No. 6,082,358 a counter attached to the base of the canister is disclosed where the counter counts when a certain force is reached. The counter is attached to the canister but does not require alignment with the actuator body since it relies on the force of the spring in the counter mechanism being less than the force of the spring in the metering valve, such that the counting occurs before firing. A problem with such a counter is that the force of the counting spring and the metering valve need to be accurately controlled, and may vary with use.
However positioning the counter on the base of the canister would be preferred since the visibility of the counter would be good and furthermore the counter could be securely fixed to the canister.
In this text we will refer to a fire point, which is the amount of compression of the canister that is necessary for delivering a dose of medicament and a count point, which is the amount of compression of the canister that is necessary for affecting the counter to count one dose.